In the production of mechanical pulps from vegetable raw material, such as wood chips, for many end uses it is necessary to effect removal of resin from the pulp. The raw material itself contains resin, and there is no opportunity for removal of the resin by chemicals such as in the production of kraft pulp. Resin is an undesirable component of the final pulp, and constitutes a complex mixture of components that are present in the wood chips, or like raw material, enclosed in ray cells, parenchyma cells, and resin ducts. Among the components of resin are steroids, waxes, glycerides, resin acids, terpenes, and fatty acids.
When resin is present in mechanical pulp, it gives a smell and taste to food packed in cardboard made from the pulp (especially cardboard for containing liquids), and since the resin is of a hydrophobic nature and makes the cellulosic material fibers of the pulp hydrophobic, the pulp is undesirable for uses such as tissue pulp and fluffed pulp where good liquid absorbency properties are necessary.
In the conventional method of removing resin from mechanical pulps, the pulp is diluted from the refiner consistency (e.g. about 25-45 percent) to a concentration of about 3-5 percent. After dilution the pulp is stored for about 15-30 minutes in a tank at 50.degree.-80.degree. C. Thereafter the pulp is pumped to a press where it is pressed to a dryness of about 30 percent. The pressate, which contains resin therein, is separated from the pulp. The pulp is again diluted and again held in another tank for about 15-30 minutes, and is again pressed. The number of dilution and pressing stages are chosen so that the desired removal of resin from the pulp can be achieved.
According to the present invention, a simplified method of resin removal is provided. According to the present invention, resin removal is accomplished along with a significant reduction of the relatively long time which is required when conventional methods are utilized. Also, according to the present invention the amount of equipment necessary to effect dilution or the like is reduced, and the energy necessary for removal can be reduced, since resin removal can be effected at a much higher consistency than in conventional procedures (e.g. at 7-20 percent, compared to 3-5 percent in conventional procedures). Also, the pressate has a significantly higher concentration of resin therein, from which additional advantages flow; and, latent properties of the pulp can inherently and simultaneously be removed.
According to the present invention a method of effecting resin removal from mechanical pulp is provided. The method comprises the steps of: (a) feeding a mechanical pulp having a consistency of about 7-20 percent to at least one high turbulent stage; (b) at the high turbulent stage, exposing the mechanical pulp at about 7-20 percent consistency to a high degree of turbulence; and (c) after step (b), dewatering the mechanical pulp to produce a pressate containing resin removed from the pulp. Typically, the time necessary to subject the pulp to the high turbulence to effect resin removal is from about 0.2-60 seconds, and usually within about 0.2-10 seconds. After dewatering the pulp can be diluted, and then the steps repeated as necessary until complete resin removal is effected. During the practice of step (b), temperature conditions of about 50.degree.-150.degree. C. (preferably about 70.degree.-120.degree. C.) are maintained, and the pulp concentration is preferably about 8-15 percent. Further, in order to enhance deresination, chemicals that improve deresination can be added during the high turbulence step.
The equipment necessary to subject the mechanical pulp to sufficiently high turbulence may comprise a fluidizing pump, and/or a screen, and/or a disc refiner, and/or a mixer, and/or a distinct turbulence generator that is disposed at the inlet of a centrifugal pump.
It is the primary object of the present invention to effect resin removal from mechanical pulps in a simple, quick, and efficient manner. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.